Centrifugal separators are well known for use within the lubrication systems of vehicle internal combustion engines as efficient means for removing very small particulate contaminants from the constantly recirculated liquid lubricant over a long period of operation. Such centrifugal separators are usually of the self-powered type, in which a separation rotor comprising a canister is supported for rotation about a rotor axis within a housing, the canister being supplied with liquid lubricant at elevated pressure along the axis and said liquid-being forced from the base of the canister (or other peripheral wall) by way of jet reaction nozzles, the reaction to said ejection causing the rotor canister and liquid within it to spin at high speed about the axis and thereby cause solid particles to migrate from the liquid passing through the canister and agglomerate on the peripheral walls thereof. The reaction nozzles are directed substantially tangentially with respect to the rotation axis, at least in a plane orthogonal to the axis, so that jets of liquid which leave the rotor canister are instantaneously tangential to the fastly spinning rotor.
It will be appreciated that the efficiency of separation is inter alia dependant upon the quantity of liquid lubricant passed therethrough in a given time and the time for which the liquid remains therein in passing through, and also upon the rotation speed of the rotor canister and contained liquid, which is in turn dependant upon the pressure drop between supply and housing and the dimensions of the nozzles, within the constraints of such nozzle dimensions/pressure drops providing sufficient torque to overcome resistance to commencement of, and continuation of, rotation.
To this end, it is commonplace to have the rotor canister divided internally by way of a radially inwardly extending partition wall which defines an outflow chamber in the vicinity of the reaction nozzles that is distinct from a separation chamber in which said particulate contaminants are separated from the input liquid and retained; the outflow chamber and reaction nozzles are protected from said separated contaminants by a transfer aperture between the chambers radially inwardly of the partition wall, that is, surrounding the rotation axis.
It is also known to have formed in the end wall of the rotor canister that bounds the separation chamber, usually the upper wall, an array of radially extending embossed ribs which by their axial extent or length relative to the wall provide strengthening for the canister wall against elevated internal pressures and also provide shallow troughs between adjacent ribs whereby liquid which enters the canister near to said wall can be accelerated by the ribs both in a circumferential direction, that is, the direction about the axis in which the ribs are travelling, and in a radial direction towards the outer peripheral side wall of the canister where circumferential speeds are higher and centrifugal separation forces higher, although this achieves less than satisfactory results in practice.
Such traditional designs of rotor canisters, whilst structurally simple and cheap to manufacture (particularly relevant when desired as single-use, throw-away, items) are operated below optimum efficiency in terms of separation. Patent specification No WO 96/22835 summarises such a typical centrifugal separator rotor canister structure and disadvantages thereof, inter alia, notwithstanding the presence of such radial ribs at the end wall the tendency for injected liquid to respond to the radial pressure gradient and flow lengthwise (axially) along a `short circuit` path close to the rotation axis rather than at radially outer regions where circumferential speeds and separation forces are stronger, before describing arrangements aimed at overcoming such disadvantages by way of structural elements within the canister that constrain the liquid to flow by way of a more radially outward part of the canister space.
The above mentioned specification particularly describes separator rotor canister arrangements in which the liquid is injected into the canister separation chamber from the rotation axis towards one (upper) end thereof and is passed to the outflow chamber at the other end thereof, also close to the rotation axis, but the separation chamber contains a structure including a stack of spaced cones, by way of which liquid can flow radially inwardly towards said axial region, and, between the structure and end of the chamber an array of radially outwardly divergent channels defined by way of a circular array of axially directed, radially extending vanes, formed either as inserts adjacent the chamber and/or structure end wall or as ribs pressed from or into the end wall of the container.
The channels defined between such vanes accelerate the liquid that is newly injected in a substantially radial direction into the chamber radially outwardly against the naturally elevated pressure associated with rotation, creating a flow path to the radially outer wall of the canister/separation chamber from where the liquid of said flow can pass between cones of the structure to join the axial flow path adjacent the rotation axis for passage to the outflow chamber. Such a canister is intended to effect an improvement in separation efficiency by constraining the injected liquid to flow at a variety of radial distances from the rotation axis at different circumferential speeds along a tortuous path, said tortuous path increasing the dwell time of the liquid within the canister and thereby improving the opportunity for contaminants to separate from the liquid flow and deposit on any suitable surface within the canister.
However, as the above mentioned specification points out, such radial acceleration of the liquid introduced into the separation chamber near to the rotation axis and upper end of the chamber is achieved at the expense of removing from the rotation canister energy that contributes to its rotation speed and separation efficiency.
Therefore, in conjunction with such radially extending acceleration vanes and acceleration chambers defined thereby, the arrangements described feature an axially extending stack of cones separated from each other in the axial direction by radially extending ribs or vanes which are stated to be acted upon by the radially inwardly returning liquid to return energy to the rotating system whereby the rotation rate of the rotor canister does not suffer. The rotor canister constructions thus described are complex internally in requiring an efficient high-pressure generating radial acceleration system and a separation cone structure which must in part recover energy expended upon said radial liquid acceleration.